Nano-sized materials may constitute key elements for advanced functional devices due to their size sensitive electrical, optical, magnetic and chemical properties. To integrate nano-structures into real devices, it is desirable to form them with controlled size and distribution on substrates compatible with current fabrication technologies.
As an example, magnetic recording technology has been driven by a strong demand for faster access speeds and for higher data storage density. To meet this demand, the areal density (i.e. the number of bits that can be stored per unit area) of magnetic hard disk data storage devices has been increasing rapidly year on year since 1991. It has been predicted that current media will encounter a physical limit which will prevent, or at least make very difficult, the manufacture of high-density magnetic storage media with an areal density of over 200-300 Gbit/in2. Currently used thin film magnetic recording media typically consist of a plurality of small, single-domain magnetic grains, which are magnetically isolated from one another. For an acceptable media signal-to-noise ratio, each recording bit must contain a number of magnetic grains. Typically several tens to several hundreds of magnetic grains in each written bit are used, with each grain having a diameter in the range of 10-20 nm or so. To increase the storage density beyond that provided by these current media, the size of the multigrain bits (and therefore each grain in each bit) must be reduced yet further. However, if the grain size is too small, the magnetization applied to the bit cannot be retained against thermal decay due to the very small energy barrier height. As a consequence, applied magnetization would be able to switch easily, and thus recorded data could be lost. To avoid this limit it is desired to use patterned media in high-density magnetic storage media with an areal density of from 100 Gbit/in.2 to tens of Tbit/in.2. Patterned media, in which each bit has several grains, help control thermal stability because the volume of a whole bit can be much larger than the volume of a single grain. Larger bits can be strongly exchange-coupled, more clearly defined and less likely to flip their magnetic state. However, no method exists which can readily produce three dimensionally ordered bits and hence these have yet to be shown possible.
When the size of magnetic particles is reduced to a few tens of nanometers, they exhibit a number of outstanding physical properties such as giant magnetoresistance, superparamagnetism, large coercivities, high Curie temperature and low magnetization saturation as compared to corresponding bulk values. Due to the realization of these outstanding physical properties upon size reduction, self-assembled magnetic nanodots and nanorods within a crystalline, matrix of another composition could bring out revolutionary advances in applications.
Semiconductor lasers are key components in a host of widely used technological products, including compact disk players and laser printers, and they will play critical roles in optical communication schemes. The basis of laser operation depends on the creation of nonequilibrium populations of electrons and holes, and coupling of electrons and holes to an optical field, which will stimulate radiative emission. It has been predicted that there are many advantages of using quantum wells as the active layer in such lasers. The ensuing carrier confinement and nature of the electronic density of states should result in more efficient devices operating at lower threshold currents than lasers with bulk active layers. In addition, the use of a quantum well, with discrete transition energy levels dependent on the quantum well dimensions (thickness), provides a means of “tuning” the resulting wavelength of the material. The critical feature size-in this case, the thickness of the quantum well-depends on the desired spacing between energy levels. Even greater benefits have been predicted for lasers with quantum dot active layers. It was also predicted in the early 1980s that quantum dot lasers should exhibit performance that is less temperature-dependent than existing semiconductor lasers, and that will in particular not degrade at elevated temperatures. Other benefits of quantum dot active layers include further reduction in threshold currents and an increase in differential gain—that is, more efficient laser operation. However, a broad distribution of sizes “smears” the density of states, producing behavior similar to that of bulk material. Thus, the challenge in realizing quantum dot lasers with operation significantly superior to that shown by quantum well lasers is that of forming high quality, uniform quantum dots in the active layer. Once again, self-assembled nanodots within a crystalline, matrix of another composition will behave as ordered quantum dots and could bring out revolutionary advances in these applications.
Magnetic oxide perovskites are well known due to the phenomenon of colossal magnetoresistance found in these materials. The 100% spin-polarization of half-metallic magnetic perovskites and the ability to epitaxially incorporate them into single crystal all-oxide heterostructures are important in spintronics applications. Neither has three dimensional ordering of such complex oxide ceramic materials been demonstrated in nano-size, nor has their incorporation into a crystallographically single crystal matrix.
For electronic devices, an ordered array of three dimensional nanodots and nanorods promises to extend device physics to full two- or three-dimensional confinement (quantum wires and dots). Multidimensional confinement in these low dimensional structures has long been predicted to alter significantly the transport and optical properties, compared to bulk or planar heterostructures. More recently, the effect of charge quantization on transport in small semiconductor quantum dots has stimulated much research in single-electron devices, in which the transfer of a single electron is sufficient to control the device. The most important factor driving active research in quantum effect is the rapidly expanding semiconductor band-gap engineering capability provided by modern epitaxy. Possible applications include spin transistors and single electron transistors. Other possible applications of three dimensionally ordered nanodots and nanorods include potential applications in optoelectronics and sensors. For example, an array of luminescent ordered nanodots within a transparent matrix can be used for devices using the photoluminescence effect.
The development of biaxially textured, second-generation, high temperature superconducting (HTS) wires is expected to enable most large-scale applications of HTS materials, in particular electric-power applications. Second-generation HTS conductors or “coated-conductors” comprise a flexible metallic substrate upon which several buffer layers and then the superconducting layer is deposited. The key goal is to have a biaxially textured superconducting layer so that few high-angle, weakly conducting grain boundaries are present. This is accomplished by epitaxial formation of the superconducting layer on biaxially textured oxide surfaces deposited upon the flexible metallic substrate. Three techniques have been developed to accomplish this—ion-beam assisted deposition (IBAD) of biaxially textured buffers on polycrystalline alloy substrates, epitaxial deposition of buffer multilayers on rolling assisted, biaxially textured substrates (RABiTS), and inclined substrate deposition (ISD) of buffers on polycrystalline alloy substrates. For epitaxial YBCO on substrates fabricated using all three techniques, the “inter-granular” critical current density is enhanced due to suppression of weak-links at grain boundaries. However, for practical application of HTS materials, the in-field performance or the “intra-granular” critical current density, also needs to be enhanced further. For many potential applications, high critical currents in applied magnetic fields are required. This is especially so for electric power applications of HTS materials as well as for military applications. For example, the underground transmission cable application requires critical current per unit width, Ic, greater than 300 A/cm in self-field; for military applications, an Ic greater than 100 A/cm and an engineering critical current density, JE, greater than 15 kA/cm2 at 65 K, 3 T and at all applied field orientations, is required; and for rotating machinery such as motors and generators, a JE of 30-40 kA/cm2 at 55-65 K, in operating fields of 3-5 T, and at all applied field orientations, is required. The phrase “total engineering critical density” is implied to include the thickness of the stabilizer layer as well.
It is well known that defects within superconducting materials can pin the magnetic flux lines so that large currents can flow through the materials in the presence of high applied magnetic fields. However, in order for the defects to be effective in pinning the flux, their size, density and geometry needs to be appropriate. Defects such as oxygen vacancies, twin boundaries, and dislocations form naturally inside films and act as pinning centers. However, the density of these naturally formed defects is not high enough to provide the necessary performance requirements for the various applications in question. To increase the density of defects for effective pinning, there have been extensive studies on introducing artificial pinning defects. Among these, linear defects such as the columnar defects produced via heavy ion irradiation have proved to be the most effective. Such columnar defects can be generated by irradiating high temperature superconducting materials with heavy-ions significantly enhance the in-field critical current density. These columnar defects leave amorphized damage tracks in the superconductor. Hence, for over a decade scientists world-wide have sought means to produce such columnar defects in HTS materials without the expense and complexity of ionizing radiation. This approach, however, is not practical for scale-up as it is not only too expensive but can render the metallic substrate radioactive.
Many electrical, electronic, optical, magnetic, electromagnetic and electro-optical devices require single crystal-like device layers with few defects within the device layer. This can be accomplished by epitaxial growth of these devices on lattice-matched, single crystal substrates. These substrates however cannot be fabricated in long lengths or in large area and essentially limited to sizes of about a foot in length and diameter at best. Hence, a variety of artificially fabricated, single crystal substrates have been developed. Among them, an important class of substrates is known as rolling assisted, biaxially textured substrates (RABiTS). Biaxial texture in a substrate refers to situation when all the grains in a polycrystalline substrate are aligned within a certain angular range with respect to one another. A polycrystalline material having biaxial texture of sufficient quality for electromagnetic applications can be generally defined as being characterized by an x-ray diffraction phi scan peak of no more than 20° full-width-half-maximum (FWHM) and a omega-scan of 10° FWHM. The X-ray phi-scan and omega-scan measure the degree of in-plane and out-of-plane texture respectively. An example of biaxial texture is the cube texture with orientation {100}<100>, wherein the (100) crystallographic plane of all grains is parallel to the substrate surface and the [100] crystallographic direction is aligned along the substrate length. Other suitable definitions have also been set forth in varying terms. It is helpful to review some of the prior work that the present invention builds upon. The entire disclosure of each of the following U.S. patents is incorporated herein by reference: U.S. Pat. Nos. 5,739,086; 5,741,377; 5,846,912; 5,898,020; 5,964,966; 5,958,599; 5,968,877; 6,077,344; 6,106,615; 6,114,287; 6,150,034; 6,156,376; 6,151,610; 6,159,610; 6,180,570; 6,235,402; 6,261,704; 6,270,908; 6,331.199; 6,375,768, 6,399,154; 6,451,450; 6,447,714; 6,440,211; 6,468,591, 6,486,100; 6,599,346; 6,602,313, 6,607,313; 6,607,838; 6,607,839; 6,610,413; 6,610,414; 6,635,097; 6,645,313; 6,537,689, 6,663,976; 6,670,308; 6,675,229; 6,716,795; 6,740,421; 6,764,770; 6,784,139; 6,790,253; 6,797,030; 6,846,344; 6,782,988; 6,890,369; 6,902,600; 7,087,113. Moreover, there are other known routes to fabrication of biaxially textured, flexible electromagnetic devices known as ion-beam-assisted deposition (IBAD) and inclined-substrate deposition (ISD). IBAD processes are described in U.S. Pat. Nos. 6,632,539, 6,214,772, 5,650,378, 5,872,080, 5,432,151, 6,361,598, 5,872,080, 6,756,139, 6,884,527, 6,899,928, 6,921,741 and these show that a {100}<100>, cube-textured surfaces can be obtained using this process; ISD processes are described in U.S. Pat. Nos. 6,190,752 and 6,265,353; all these patents are incorporated herein by reference. In the ISD process, the resulting substrate has rotated cube texture and the rotation can be as high as 40-45°. In the IBAD and ISD processes a flexible, polycrystalline, untextured substrate is used and then a biaxially textured layer is deposited on this substrate.
Large-area and flexible single crystal substrates have also been fabricated as reported in U.S. Pat. No. 7,087,113 by Goyal. This patent is also incorporated herein by reference.